In a statement posted on the website searchingforkeith.com, which is run by Mrs Johnson's son, Alan Bennett, the family said that the fight to find Keith's body would continue in her memory.

The statement in full said: “Winnie Johnson, the mother of Keith Bennett, died this morning at 12.35am, aged 78. She was a much-loved mother, grandmother and great-grandmother and is survived by one younger brother.

“Winnie fought tirelessly for decades to find Keith and give him a Christian burial.

“Although this was not possible during her lifetime we, her family, intend to continue this fight now for her and for Keith.

“We hope that the authorities and the public will support us in this.

“We would like to express our deep gratitude to all the medical staff who cared for Winnie during her illness, and also the public for their good wishes and support over the years.

“We hope that everyone will understand, however, that we would like to grieve for her in private, both at this time and during her funeral.”

Winnie Johnson, shown speaking to the M.E.N. in December 2011:

John Ainley, Mrs Johnson's solicitor, said today: “Winnie Johnson, mother of the last missing Moors murder victim Keith Bennett, has died without discovering her son's whereabouts.

Winnie, 78, passed away peacefully at a hospice where she was being cared for with cancer.

“It is a terrible tragedy and a distressing time. She will be sadly missed and was much-loved by her family. Winnie insisted that Brady is the only person who could put her mind at rest and give her the chance to give Keith a decent burial before she passed on.

“She was convinced that Brady knew where her son was buried and she wanted the search to continue to find Keith. However, despite her personal appeals directly to Brady and via my office, Brady haspersistently ignored the wishes of a tragic grieving mother.

“She died without knowing Keith's whereabouts and without the opportunity to finally put him at rest in a proper grave. It is truly a heartbreaking situation that this opportunity has now been lost.

“Winnie's health deteriorated in the last few days but she died with a steadfast conviction that Ian Brady could resolve the situation.”

Mr Ainley said Mrs Johnson had been in the hospice for 'some weeks'.

And he described her as 'a most courageous lady' who was 'the driving force' behind the long search for her son.

Mr Ainley added: “She always thought that she would find Keith but without information from Ian Brady, that became most difficult.”

David Kirwan, a solicitor who represented Mrs Johnson for three years, paid tribute to her as "a mother".

The lawyer, who twice met the serial killer to discuss the possibility of him being taken back to the Moors to help the search, said: "It is an absolute tragedy.

"I spent approximately four years helping Winnie in her campaign but she spent 48 years searching for her son.

"I would sum her up with one word, a mother.

"This is what a parent will do, look at the McCanns, their search has never stopped (for daughter Madeleine)."

Mr Kirwan said he was optimistic about news of a letter from Brady which could divulge Keith's burial site.

"I believe that in all of us there is a human trait, the desire for the confessional, to make your peace," said Mr Kirwan.

However, Brady is such a control freak with such a venomous hatred of the authorities, any clues he provides will be in riddle form, Mr Kirwan said.

"He thinks he is cleverer than everybody else, thinks he is superior to everybody else."

News of Mrs Johnson's death today comes as police continue to investigate claims of a letter said to have been written by killer Ian Brady and addressed to her which could pinpoint the location ofKeith's shallow grave on Saddleworth Moor, above Oldham.

An investigation was launched after Brady's mental health advocate Jackie Powell revealed the killer gave her a sealed envelope to pass to Mrs Johnson after he dies.

Mrs Johnson's legal team confirmed she was not told of the development.

Ms Powell, 49, who was appointed Brady's representative under the Mental Health Act in 1999, was arrested on suspicion of preventing the burial of a body without lawful exercise yesterday morning and was bailed pending further inquiries.

Police have seized paperwork from Ms Powell's home near Llanelli in south Wales and were continuing to sift through it today in a bid to find the letter.

But senior police chiefs admit that they do not know whether the it true or 'simply a ruse'.

Martin Bottomley, head of GMP's cold case unit, also issued a direct plea to Brady.

He said: “I would implore him at this extremely sad time and knowing that Winnie has died not knowing where Keith is buried to at last do the decent thing and tell us where he is.”

Mr Bottomley added: “Our thoughts are with Winnie’s family at this very difficult and sad time. Winnie spent the majority of her life courageously fighting to get justice for Keith.

“All she wanted was to know where Keith was buried so she could layhim to rest.

“Sadly, almost all of her life, she has had to live with the knowledge that Ian Brady refused to show compassion and do the right thing and disclose where he had buried her little boy.

“Winnie was a tenacious and courageous woman and I wish that in her lifetime, she could have found where Keith was buried. The only comfort we can take is that she is now at peace with the little boy she missed so much.

“We are continuing to examine the documentation we seized to establishwhether or not this letter even exists.”

Brady and Myra Hindley, who died in prison aged 60 in 2002, murdered five children in the 1960s and buried them in shallow graves on Saddleworth Moor.

Keith was abducted from the street as he walked to his gran's house in Longsight in 1964 then murdered. His body is the only one not to have been found and both Brady and Hindley repeatedly refused to reveal its location.

Brady and Hindley were both convicted in 1966. Brady was jailed for life for the murders of John Kilbride, 12, Lesley Ann Downey, 10 and Edward Evans, 17.

Hindley was caged for life for killing Lesley Ann and Edward and shielding Brady after John’s murder. In 1987, the pair admitted killing Keith Bennett and Pauline Reade.

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